Question 39547
Domain means all allowable values of x.  In
y = sqrt(x^2 - 25)
Notice that the radicand, x^2 - 25, cannot be less than zero...so if we set
x^2 - 25 ≥ 0
we can determine the domain...
x^2 ≥ 25
|x| ≥ 5
Thus all values satisfying the above are in the domain.
So x must be 
x ≥ 5 or x ≤ -5